1. Field of the Invention
This invention relates to a radiation image read-out method and apparatus. This invention particularly relates to a radiation image read-out method and apparatus, wherein light emitted by a stimulable phosphor sheet is detected with a line sensor or an area sensor.
2. Description of the Prior Art
It has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a radiation image of an object, such as a human body, is recorded on a stimulable phosphor sheet, which comprises a substrate and a layer of the stimulable phosphor overlaid on the substrate. Stimulating rays, such as a laser beam, are deflected and caused to scan pixels in the radiation image, which has been stored on the stimulable phosphor sheet, one after another. The stimulating rays cause the stimulable phosphor sheet to emit light in proportion to the amount of energy stored thereon during its exposure to the radiation. The light emitted successively from the pixels in the radiation image having been stored on the stimulable phosphor sheet, upon stimulation thereof, is photoelectrically detected and converted into an electric image signal by photoelectric read-out means. The stimulable phosphor sheet, from which the image signal has been detected, is then exposed to erasing light, and radiation energy remaining thereon is thereby released.
The image signal, which has been obtained from the radiation image recording and reproducing systems, is then subjected to image processing, such as gradation processing and processing in the frequency domain, such that a visible radiation image, which has good image quality and can serve as an effective tool in, particularly, the efficient and accurate diagnosis of an illness, can be obtained. The image signal having been obtained from the image processing is utilized for reproducing a visible image for diagnosis, or the like, on film or on a high resolution cathode ray tube (CRT) display device. The stimulable phosphor sheet, from which residual radiation energy has been released with the erasing light, can be used again for the recording of a radiation image.
Novel radiation image read-out apparatuses for use in the radiation image recording and reproducing systems described above have been proposed in, for example, Japanese Unexamined Patent Publication Nos. 60(1985)-111568, 60(1985)-236354, and 1(1989)-101540. In the proposed radiation image read-out apparatuses, from the point of view of keeping the emitted light detection time short, reducing the size of the apparatus, and keeping the cost low, a line light source for irradiating linear stimulating rays onto a stimulable phosphor sheet is utilized as a stimulating ray source, and a line sensor comprising a plurality of photoelectric conversion devices arrayed along the length direction of a linear area of the stimulable phosphor sheet, onto which the stimulating rays are irradiated by the line light source, is utilized as photoelectric read-out means. Also, the proposed radiation image read-out apparatuses comprise scanning means for moving the stimulable phosphor sheet with respect to the line light source and the line sensor and in a direction, which is approximately normal to the length direction of the linear area of the stimulable phosphor sheet.
FIGS. 6A, 6B, and 6C are explanatory views showing relationship between a line width of light emitted by a stimulable phosphor sheet and a photoelectric conversion device constituting a conventional line sensor. In FIG. 6A, a beam width (a line width) of light M emitted linearly (i.e., in a linear pattern extending along a direction normal to the plane of the sheet of FIG. 6A) by a stimulable phosphor sheet 50 is represented by d.sub.M. FIGS. 6B and 6C show the distribution of the intensity of the emitted light M along the line width direction. As illustrated in FIG. 6B, in cases where the emitted light M is collected by a line sensor, in which a light receiving width d.sub.P of each photoelectric conversion device is smaller than the line width d.sub.M, the light collecting efficiency cannot be kept high. Also, as illustrated in FIG. 6C, in cases where the emitted light M is collected by a line sensor, in which the light receiving width d.sub.P of each photoelectric conversion device is approximately equal to the line width d.sub.M, the light collecting efficiency can be kept high. However, in such cases, since the size of each pixel is large, the problems occur in that the resolution cannot be kept high. (The same problems occur also when each photoelectric conversion device has a rectangular shape such that the length along the line width direction may be larger than the length in the direction along which the line extends.)
The emitted light M has the intensity distribution shown in FIGS. 6B and 6C since the line width of the stimulating rays L becomes large before impinging upon the stimulable phosphor sheet 50, since, as illustrated in FIGS. 3A and 3B, the stimulating rays L of a line width d.sub.L (&lt;d.sub.M) having entered into the stimulable phosphor sheet 50 are scattered in the stimulable phosphor sheet 50, and since the emitted light M having occurred in the stimulable phosphor sheet 50 is scattered in the stimulable phosphor sheet 50 before being radiated out of the surface of the stimulable phosphor sheet 50.